They were dicing with death. The five young boys skipped along the tracks, oblivious of the lethal currents coursing through the steel rails between their feet.

In a few minutes, the next passenger train travelling between Leeds and Bradford would be thundering past at more than 60mph. The 10-year-olds standing directly in the path of the train would have no chance.

In the sky above, the Railtrack watchers were livid. Less than a week earlier, officials had toured every local school to warn children of the dangers of rail trespass.

Hovering more than 1,000ft above the ground, photographer Paul Ward strained to record the movement below. His pictures would later be handed over to the police to identify offenders.

After a few minutes wandering along the track, the boys had stepped off to grab their bicycles from a grass verge and vanished.

Operation Skyhawk had just logged another track incursion. At a cost of more than £4,000 a day, Railtrack and the British Transport Police have deployed the aircraft and photographer to monitor the worst areas in Britain for rail vandalism and trespassing.

With more than 46,000 recorded incidents a year, experts say it is only a matter of time before those who risk their own lives to take short cuts or damage the railways cause a major disaster.

Minutes after the boys had left the Leeds-Bradford line, pilot Bob Hodgson's radio crackled into life again. It was an urgent message from a police patrol car: 'Divert towards Doncaster - kids have put a wheelie-bin across the high speed line.'

After banking sharply, the helicoptor, with The Observer on board, raced south to a small, unmanned station between Wakefield and Doncaster called Fitzwilliam.

A Virgin Trains driver had reported a gang of youths on the station platform with a wheelie-bin. By the time the Railtrack helicoptor arrived, the bin had already been placed on the line.

The Leeds area is one of a number of 'hotspots' included as part of Operation Skyhawk. With more than 31 incidents of criminal trespass last year, the region is now second in the nation's rankings.

Even a special edition of the children's story favourite Thomas the Tank Engine , complete with a safety theme, to be published later this month, may not be enough to deter vandals and trespassers.

Railtrack figures show more than 16,000 trespass offences were reported last year. Most were committed by children aged between 11 and 16. The figure included 700 'near misses' with trains, and the deaths of 64 people - among them eight children - who strayed on to lines, a rise of 25 per cent on 1999.

But officials believe this figure is almost certainly dwarfed by potentially 'millions' of undetected or unreported incidents each year including those who take short cuts to work or school every day. The culprits are identified in almost a quarter of the cases reported to police, but only 8 per cent result in prosecutions.

However, more worrying for rail officials is the rise in the number of incidents where obstacles were placed on lines. Last year saw 30,000 acts of vandalism including a 10 per cent rise in the number of incidents with the potential gravely to endanger safety. Eight of the 10 worst areas for vandalism in the country are in the Midlands.

Last year there were only 302 criminal convictions for trespass and vandalism and one custodial sentence. Aidan Nelson, policy and standards director of Railtrack's safety arm, said: 'Obstructions put on the line have the potential to derail a train with catastrophic consequences.'

Railtrack estimates that the overall cost to the rail industry in terms of damage to the network and delays caused by incidents is £27 million a year. But the company admits it has failed to meet its own targets for reducing trespass and vandalism on the railways.

So-called 'malicious acts' cause more than 55 per cent of rail accidents every year.

Railtrack admitted that last year's Hatfield crash had exacerbated the problem. Emergency repairs around the country have led to more material delivered to the trackside being used as missiles by vandals. The company has also admitted it is struggling to reach its goal of reducing deaths of trespassers.

This was clearly lost on the children wandering along the line near Leeds last week. Tony Prankett, of Railtrack, said: 'A train would have sounded its horn long and loud and the kids would have moved a couple of inches and stuck two fingers up at it.'

Despite the increased vigilance and safety advice, a survey by Railtrack found children were showing 'less understanding of the risks' than they had the previous year.

Operation Skyhawk will resume in the summer holidays, targeting the North-East and South Wales and backed up by police patrols on the ground.